1. Field of the Invention
The present invention relates to plasma processing apparatuses and electrostatic attract-and-hold vacuum chucking methods employed therein, and in particular to plasma processing apparatuses electrostatically attracting and holding semiconductor wafers to fix the semiconductor wafers and electrostatic attract-and-hold vacuum chucking methods employed therein.
2. Description of the Background Art
In recent years, electrostatic chuck technology has been increasingly used for apparatuses which process semiconductor wafers as desired, such as plasma etching apparatuses, plasma film-forming apparatuses. Electrostatic chuck technology can prevent deposition of foreign matters at the perimeter of a semiconductor wafer that have been conventionally often produced at a wafer clamp clamping the perimeter of the semiconductor wafer. This ensures that the most outer peripheral portion of a semiconductor device fabricated on the semiconductor wafer can be provided as a product to increase yield. Electrostatic chuck technology is a technology that can be utilized for various semiconductor manufacturing apparatuses in the future.
Referring to FIG. 1, a conventional plasma processing apparatus 60 which employs electrostatic chuck technology includes a vacuum chamber 21 blocking the external atmosphere from the internal for maintaining the internal atmosphere.
Vacuum chamber 21 includes a lower electrode 24, a dielectric film 23 formed on a surface of lower electrode 24 to attract a semiconductor wafer 22 through electrostatic force, a gas supply port 25 for introducing a desired gas into vacuum chamber 21 from e.g. a gas cylinder (not shown), an upper electrode 26 arranged opposite to lower electrode 24 for diffusing the gas introduced via gas supply port 25 to introduce the gas into vacuum chamber 21 and also functioning as an electrode, an exhaust port 27 provided to exhaust the gas in the vacuum chamber 21 by means of a vacuum pump (not shown), and an insulator 33 formed on lower electrode 24 to maintain the insulation between lower electrode 24 and the gas in vacuum chamber 21.
Plasma processing apparatus 60 also includes an electrostatic chuck power supply 31 for applying a desired voltage to dielectric film 23 via lower electrode 24, a control signal unit 32 receiving a value of an electrostatic chuck voltage Vs (described hereinafter) stored in a processing-condition memory unit 62 described hereinafter to control a voltage output from electrostatic chuck power supply 31 and thus apply electrostatic chuck voltage Vs from electrostatic chuck power supply 31 to lower electrode 24, a high-frequency power supply 29 for applying high-frequency electric power to lower electrode 24, a high-frequency cutting filter 30 provided to prevent the high-frequency electric power from sneaking from high-frequency power supply 29, and a matching transformer 28 for achieving the matching/integrity between high-frequency power supply 29 and lower electrode 24.
A desired gas introduced into vacuum chamber 21 is electromagnetized by high-frequency power supply 29 to produce a plasma 34.
Plasma processing apparatus 60 also includes a processing-condition memory unit 62 for storing the conditions for producing plasma 34 desired, such as gas flow, the pressure in vacuum chamber 21, the magnitude of high-frequency electric power (referred to as "processing conditions" hereinafter), and the voltage applied from electrostatic chuck power supply 31 to lower electrode 24, or electrostatic chuck voltage Vs.
A plasma 34 producing operation effected in plasma processing apparatus 60 will now be described briefly and electrostatic attract-and-hold vacuum chuck operation will then be described.
Plasma Producing Operation
Semiconductor wafer 22 is transported into vacuum chamber 21 via a transport device (not shown) and mounted on lower electrode 24 with dielectric film 23 interposed therebetween. Depending on the processing conditions stored in processing-condition memory unit 62, a predetermined amount of gas is introduced from gas supply port 25 via upper electrode 26 into vacuum chamber 21. Simultaneously, a predetermined amount of gas is exhausted from exhaust port 27. Thus, the pressure inside vacuum chamber 21 is adjusted to have the value of a pressure determined by the processing conditions. Then, high-frequency power supply 29 applies high-frequency electric power to lower electrode 24 via matching transformer 28. Associated with the application of high-frequency electric power, plasma 34 is produced inside vacuum chamber 21. Then, desired processes, such as etching, film-forming, are applied to semiconductor wafer 22.
Electrostatic Attract-and-Hold Vacuum Chucking Operation
When semiconductor wafer 22 is mounted on dielectric film 23 and plasma 34 is produced in vacuum chamber 21, an equivalent circuit, such as shown in FIG. 2, is formed.
The equivalent circuit shown in FIG. 2 includes electrostatic chuck power supply 31 having one end connected to the ground and the other end connected to lower electrode 24 for applying electrostatic chuck voltage Vs to lower electrode 24, dielectric film 23 formed on lower electrode 24, semiconductor wafer 22 mounted on dielectric film 23, and an equivalent plasma resistance 70 having one end connected to semiconductor wafer 22 and the other end connected to the ground, and formed of plasma 34.
When electrostatic chuck power supply 31 applies negative (-) direct current voltage to lower electrode 24, positive (+) and negative (-) electric charges are induced at an interface between lower electrode 24 and dielectric film 23 and between dielectric film 23 and semiconductor wafer 22. As a result, the attraction referred to as Coulomb force or Johnsen-Rahbeck force is caused between semiconductor wafer 22 and dielectric film 23 and semiconductor wafer 22 is thus attracted onto dielectric film 23. Thus, conventional plasma process apparatus 60 can reliably attract semiconductor wafer 22 onto dielectric film 23 when the characteristics of plasma 34 formed are constant.
In plasma processing apparatus 60, the difference between the electron current and iron current that flow onto semiconductor wafer 22 causes a self-bias voltage Vdc. The value of self-bias voltage Vdc varies depending on the condition of plasma 34.
Referring to FIG. 3, the relation represented as equation (1) is established between self-bias voltage Vdc, a voltage V1 caused between semiconductor wafer 22 and dielectric film 23, and electrostatic chuck voltage Vs: EQU Vs=V1+Vdc (1)
As has been mentioned above, the value of self-bias voltage Vdc varies depending on the condition of plasma 34. In conventional plasma processing apparatus 60, however, the value of electrostatic chuck voltage Vs is fixed. Accordingly, for conventional plasma processing apparatus 60, the value of voltage V1 decreases as the value of self-bias voltage Vdc increases. Thus, the force to attract and hold semiconductor wafer 22 is reduced this disadvantageously.
FIG. 4 shows respective experiment results of a self-bias voltage Vdc and a minimal voltage Vmin required to attract and thus hold wafer 22 of 8".phi. on dielectric film 23 when the high-frequency electric power output from high-frequency power supply 29 is varied. Minimal voltage Vmin is a voltage applied from electrostatic chuck power supply 31 to lower electrode 24 to attract and hold semiconductor wafer 22 on dielectric film 23. The graph shows that as that self-bias voltage Vdc has a more negative value, minimal voltage Vmin also has a more negative value. For example, when electrostatic chuck voltage Vs is set at -450V, it is understood that semiconductor wafer 22 can be attracted and held for a high-frequency electric power of no more than 400 W whereas semiconductor wafer 22 cannot be attracted or held for a high-frequency electric power of 500 W. It is thus understood that determining the value of electrostatic chuck voltage Vs depending on self-bias voltage Vdc is important in stabilizing the force to attract and hold semiconductor wafer 22.
For a plasma processing apparatus disclosed in Japanese Patent Laying-Open No. 8-124913, a computer is employed to observe self-bias voltage Vdc. Depending on the value of self-bias voltage Vdc observed, the value of electrostatic chuck voltage Vs is corrected and thus applied to an electrode to stabilize the force to attract and hold a semiconductor wafer.
There is a time delay caused, however, in observing self-bias voltage Vdc and then feeding the observed value back to electrostatic chuck voltage Vs. Furthermore, the value of self-bias voltage Vdc varies from time to time, since the plasma is not stable at the start of process.
Thus, electrostatic chuck voltage Vs corrected can fail to provide the voltage sufficient to attract and hold a semiconductor wafer. This results in a disadvantage that the force to attract and hold the semiconductor wafer is not stabilized.